There is a need for rubber compositions, particularly rubber compositions used for tires, to have weather resistance (ozone resistance), in other words, it is needed to prevent cracking and deterioration of the rubber composition caused by atmospheric ozone. For this reason, wax is added to such rubber compositions.
Paraffin wax has been mainly used as wax to be added to rubber compositions. However, this is not preferred in view of the global environment because paraffin wax is synthesized from petroleum which is a fossil resource. In addition, while petroleum wax has excellent weather resistance, it unfortunately damages the aesthetic qualities (the appearance of tires) by discoloration caused by blooming. Further, because petroleum wax has a low polarity, when it is used in a large amount in recent fuel-economy compounds, such as silica rubber compounds, it unfortunately leads to poor fuel economy.
Hence, exemplary rubber compounds in which naturally occurring wax, such as carnauba wax, jojoba wax and rice bran wax, is used have recently been suggested (for example, Patent Literatures 1 and 2).
However, although the use of a wax having a broad carbon number distribution ensures weather resistance in a wide temperature range, the carbon number distribution (molecular weight distribution), i.e., the softening point distribution, of natural waxes varies depending on the organisms from which the waxes are derived, and not many natural waxes contain components having a wide softening temperature range. Therefore, the use of natural waxes for tires has a problem in terms of weather resistance in a wide temperature range, particularly in a low-temperature region. Furthermore, these waxes contain an ester component as the main component, have a low hydrocarbon content, and thus have poor compatibility with a rubber component (for example, low polarity rubbers such as natural rubber, butadiene rubber, and isoprene rubber), compared with petroleum-derived waxes. This creates a problem that sufficient weather resistance cannot be obtained. In addition, natural waxes are made from plant resources such as trees. This also creates problems in terms of availability and cost, i.e., productivity depends on the weather, and labor cost is high.
Meanwhile, an attempt has been made to use candelilla wax and carnauba wax in combination in order to enhance weather resistance (for example, Patent Literature 2). However, even with the use of both natural waxes, weather resistance (particularly at low temperatures) and measures against discoloration are insufficient, and an improvement has thus been desired.
Exemplary rubber compounds in which an isoprenoid extracted from, for example, plants, and the like are appropriately mixed to provide better properties are also suggested (for example, Patent Literature 3).
However, because the content of such isoprenoids in plants or microorganisms is very small, their mass production is difficult and no examples of practical applications have been reported. This shows that there have been problems in terms of availability and cost.